This invention relates to a process of producing polytrimethylene terephthalate by esterification of terephthalic acid with 1,3-propanediol, precondensation of the esterification product to obtain a precondensation product, and polycondensation of the precondensation product to obtain polytrimethylene terephthalate, wherein the 1,3-propanediol is recycled. More specifically, the present invention relates to an improvement in the recycle of the 1,3-propanediol wherein solid byproducts are converted into liquid or semi-liquid form.
The preparation of polytrimethylene terephthalate (PTT) involves the reaction of terephthalic acid (TPA) or dimethylterephthalate (DMT) and excess 1,3-propanediol (PDO) at elevated temperatures, 240 to 275xc2x0 C., optionally in the presence of an esterification catalyst such as a catalytic titanium compound, to obtain an esterification product which is usually a relatively low intrinsic viscosity PTT. This esterification product is then subjected to precondensation and finally the precondensation product is subjected to polycondensation to obtain PTT. In some processes, this is followed by solid state polymerization to increase the intrinsic viscosity of the PTT but there is a new process which can produce high intrinsic viscosity PTT without solid state polymerization.
In the PTT process, the excess PDO (which contains a number of materials which have to be removed before the PDO can be recycled) removed from the prepolymer (precondensation) and polycondensation stages can be distilled to recover purified PDO for reuse in the process. The PDO recovery typically consists of boiling and separating PDO vapor from high boiling fraction (distillate bottoms; solids and semi-solids). Further purification of PDO in a distillation column or other such means can be effected.
The instant invention is a process that chemically converts the solid and semi-solid byproducts (distillate bottoms, herein collectively described as xe2x80x9csolid byproductxe2x80x9d) into compositions that are flowable liquids or fluids at practical temperatures. These compositions, which can be homogeneous solutions or non-homogeneous suspensions, are lower in viscosity than the starting byproducts and, therefore, are easier to handle for disposal and/or recycle to the process. Preferably, these flowable liquids or fluids will have a viscosity of less than about 500 mPaxc2x7s at 100xc2x0 C.
This invention is an improvement upon the known process for polymerization of PTT by esterification of TPA or DMT with PDO, precondensation of the esterification product to produce a precondensation product, polycondensation of the precondensation product to produce PTT, and purification of the excess PDO which can then be recycled. In the PDO purification stage, the excess PDO removed from the precondensation and/or polycondensation stages is distilled to recover purified PDO for reuse in the process. The PDO containing stream is boiled and PDO is separated from the high boiling byproduct fraction (distillate bottomsxe2x80x94solid byproduct) which are the solids and semi-solids referred to above. Another step further purifies the PDO in, for example, a distillation column which can be combined with or separate from the initial fractionation.
The improvement comprises heating the solids and semi-solids (solid byproduct) in the presence of a metal catalyst which digests and converts the solid byproduct (sludge) to esters of terephthalic acid, primarily the di-PDO ester [bis(3-hydroxypropyl) terephthalate]. The metal catalyst is selected from one or more 3rd, 4th, or 5th row metal compounds from Groups 3-12 and Groups 14-15 of the Periodic Table (IUPAC 1989). Metal salts based on La, Ti, Zr, V, Cr, Mn, Ru, Co, Ni, Zn, Sn, and Sb are preferred. Catalysts based on Ti, Zn, and Sn are most preferred because of their higher reactivity , and compatibility with the PTT polymerization process, especially if the digestion product is to be recycled. The amount of catalyst is that which effects the desired conversion (xe2x80x9cdigestionxe2x80x9d) under the reaction conditions. This is generally 5 ppm (based on metal) to about 5000 ppm, preferably from about 10 ppm to about 500 ppm.
The temperature of the conversion reaction may range from about 100xc2x0 C. to about 240xc2x0 C. If the reaction takes place in a separation vessel, the recommended range is about 140xc2x0 C. to about 240xc2x0 C., the preferred range is from about 180xc2x0 C. to about 240xc2x0 C., and the most preferred range is about 210xc2x0 C. to about 230xc2x0 C. The reaction (xe2x80x9cdigestionxe2x80x9d), when carried out in a separate reaction vessel, is conducted at temperatures from about 100xc2x0 C. to about 220xc2x0 C., preferably from about 120xc2x0 C. to about 200xc2x0 C., and most preferably from about 140xc2x0 C. to about 180xc2x0 C. Reaction times can range from a few minutes to several hours. Preferably, the reaction time will be from about 5 minutes to about 24 hours, most preferably from about 10 minutes to about 2 hours. A fluid product, preferably having a viscosity of less than about 500 mPaxc2x7s at 100xc2x0 C., is produced.
In one embodiment, the excess PDO containing the byproducts are transferred to a separation vessel, i.e., distillation column, flasher, or similar vessel, where the PDO vapor is removed overhead, leaving a more concentrated solid byproduct stream which is transferred to a separate reaction vessel where the catalyst is added and the digestion (conversion reaction) takes place.
In another embodiment, the catalyst is added to the separation vessel used for PDO recovery and the digestion takes place in situ there.
In a third embodiment, the PDO recovery stage (separation vessel) is partially or completely skipped and the solid byproduct is transferred from the vacuum condenser systems of the precondensation and/or polycondensation stages (wherein 1,3-propanediol vapor is separated from solid byproduct) to a reaction vessel wherein the catalyst is added and the digestion reaction takes place.
In a fourth embodiment, the excess PDO is transferred to a separation vessel which is part of the esterification stage of the process (wherein 1,3-propanediol vapor is separated from solid byproduct) wherein the catalyst is added to the separation vessel, i.e., distillation column, flasher, or similar vessel, and the digestion reaction takes place there, preventing the formation of a large amount of solid byproduct.